A typical uninterruptable power supply (UPS) system is shown in FIG. 1. The UPS system comprises an input converter for converting electric power from an AC or DC power source. A typical UPS has an input power stage that converts the commercial AC mains or a renewable energy AC or DC source, to a DC voltage. The DC voltage is then converted by means of a converter to a controlled AC or DC voltage thus forming a power supply to electrical loads such as computers, refrigerators etc that need uninterrupted power supply. Typically, a regulated DC bus is provided between the input converter and the output converter. The DC bus is ideally suited to be connected to a DC battery. A bidirectional DC/DC converter is needed if the battery voltage differs from the DC voltage on the DC-bus.
A typical AC-UPS can handle a power failure in the AC mains, by using a battery to supply DC power to the DC bus via a DC/DC converter, so the power supplied to the AC load is not interrupted. When the AC mains is operable again, power from the AC mains can be used to recharge the battery and supply energy to the load.
In many UPS systems, one DC/DC converter is used for supplying power from the battery to the DC bus and a separate converter is used to charge the battery. The battery charging converter can either be an AC/DC converter supplied with power from the AC mains, or a separate DC/DC converter supplied with power from the DC bus.
In some applications, the input may also be a DC source, for example a renewable electrical energy source such as solar cells.
There exist many proposed circuits for one-directional and bi-directional DC/DC converters for improved efficiency such as in “Performance optimization of a High Current Dual Active Bridge with Wide operating Voltage Range”, by Krismer, Round, Kolar” published in Power Electronics Specialists Conference, 2006, “A new HE ZVZCS Bidirectional DC/DC converter for HEV 42V Power Systems”, by Kim, Han, Park, Moon published in Journal of Power Electronics, Vol. 6, No. 3, July 2006, “Bidirectional DC/DC Power Conversion using Quasi-Resonant Topology”, by Ray, published in Power Electronics Specialists Conference, 1992, “A Bidirectional DC—DC converter for renewable energy systems”, by Jalbrzykowski, Citko, published in Bulletin of the Polish Academy of Technical Sciences Vol. 57, No. 4, 2009. All of these are operated at fixed frequency and controlled by either phase-shifting gate pulses or duty cycle modulation. All of these circuits have the drawback of limited operating ranges for high efficiency operation, such as zero voltage switching (ZVS) of main switches for only a limited load range etc.
The object of the present invention is to provide a method for controlling a series resonant DC/DC converter with improved efficiency. The object is also to provide a method for controlling a series resonant DC/DC converter so that a bidirectional DC/DC converter is achieved. Hence, the series resonant DC/DC converter can be used both for supplying power from the battery to the DC bus during power failures and for supplying power from the DC bus to recharge the battery after a power failure. This reduces the number of components, and hence costs/space can be saved.